A magnet work machine for attracting metal scraps or the like by a magnet (commonly called “lifmag machine”) includes, as a main body, a hydraulic excavator including a self-propelled base machine 3 having a lower travelling body 1 and an upper slewing body 2, and a work attachment A having a boom 4 attached to the base machine 3 and an arm 5 as shown in FIG. 6. Additionally, the magnet work machine, which includes a magnet 6 mounted on a front end of the work attachment A (at a front end of the arm 5 as shown in the figure, or at a bucket not shown), carries metal scraps or the like, with the metal scraps or the like being attracted to the magnet 6.
As the magnet work machine, there is known a hybrid work machine which includes an engine, a generator motor driven by the engine, and a battery, and operates a magnet with the battery as a power supply (see Patent Literature 1).
In the known technique, electric power generated by the generator motor is transmitted to the battery to charge the battery, and the electric power stored in the battery is transmitted to the magnet to energize the magnet. On the other hand, during demagnetization, i.e., when electric power supply from the battery to the magnet is cut off in order to release charges attracted to the magnet, the electric power supplied to the magnet is recovered (regenerated) in the battery.
According to the battery-driven system, electric power supply and regeneration for attraction and release by a magnet is conducted only between the battery and the magnet. Therefore, the system has an advantage that a voltage of a main circuit (a main circuit voltage) connecting the magnet and a power supply does not have a large fluctuation.
However, the above known technique has a large disadvantage. Specifically, essential provision of a battery complicates a device configuration, thus increasing costs and space for facilities. Further, control for maintaining a battery charging rate to be appropriate for attraction and release and control for making a battery temperature constant are required to complicate the control.